Many substances increase their lengths or volumes due to thermal expansion as the temperature elevates. By contrast, materials exhibiting negative thermal expansion (hereinafter, also referred to as “negative thermal expansion materials”) are also known which rather decrease their volumes when warmed. It is known that the materials exhibiting negative thermal expansion can be used with other materials to suppress change in the thermal expansion of the materials caused by change in temperature.
For example, β-eucryptite, zirconium tungstate (ZrW2O8), zirconium tungsten phosphate (Zr2WO4(PO4)2), ZnxCd1-x(CN)2, manganese nitride, and bismuth-nickel-iron oxide are known as the materials exhibiting negative thermal expansion.
The zirconium tungsten, phosphate has large negative thermal expansivity with a linear expansion coefficient of −3.4 to −3.0 ppm/° C. in the temperature range of 0 to 400° C. and can be used in combination with a material exhibiting positive thermal expansion (hereinafter, also referred to as a “positive thermal expansion material”) to produce a low thermal expansion material.
As for method for producing the zirconium tungsten phosphate, for example, Patent Literature 1 described below has proposed a method which comprises mixing zirconium phosphate, tungsten oxide and a reaction accelerator such as MgO in a wet ball mill, and calcining the obtained mixture at 1200° C. Patent Literature 2 described below has proposed a method which comprises wet-mixing a phosphorus source such as ammonium phosphate, a tungsten source such as ammonium tungstate and a zirconium source such as zirconium chloride, followed by calcination. Non Patent Literature 1 described below has proposed, for example, a method which comprises calcining a mixture containing zirconium oxide, tungsten oxide and ammonium dihydrogen phosphate at 1200° C. However, the particle shape of the zirconium tungsten phosphate produced according to Patent Literatures 1 and 2 and Non Patent Literature 1 is a fractured shape. As far as the present inventor is aware, no spherical zirconium tungsten phosphate has been reported.